Wireless communication channels are often subject to degradation such as multi-path distortion or fading. As is known in the art, multi-path distortion is caused by a transmitted signal taking more than one propagation path from the transmitter to the receiver. Typically, the desired path between a transmitter and a receiver is the most direct path since signals traveling along this path have the lowest propagation time. Other possible propagation paths between the transmitter and the receiver are not as direct and therefore are longer than the direct path and accordingly have a greater propagation time.
In a typical communication system, a transmitter will emit a signal, the receiver will receive the direct path emitted signal and, at some time later, the receiver will also receive one or more of the non-direct path emitted signals, known in the art as multipath signals. The multipath signals are typically substantially the same as the direct path signal albeit for the time delay in reaching the receiver. Consequently, the receiver will receive multiple versions of the emitted signal. The received multipath signals may be delayed by an amount of time that causes the multipath signal of an emitted signal to interfere with the direct path signal of a subsequently-emitted signal, thereby causing intersymbol interference at the receiver.
Communication systems, which utilize Frequency Shift Keyed (FSK) signaling or other modulation schemes to convey information, often operate over communication channels that can experience these multiple delayed versions of the transmitted waveform at the receiver. While a real concern for fixed systems of transmitter and receivers, a reception signal received by a portable communication terminal such as an automobile telephone, a portable telephone, or the like, is more readily distorted because of multi-path distortion and is more difficult to compensate for in that the multi-path profile is constantly changing as the receiver moves through different geographic and atmospheric environments.
Frequency Shift Keying is an extremely mature modulation or signaling technique utilized to convey data over radio links. It is characterized as the transmission of sinusoidal waveforms of different frequencies, dependent of the data to be conveyed. the simplest form of FSK, binary FSK, sends one frequency sinusoidal when the data bit is a “0” and an alternate frequency sinusoid when the data is a “1”. On a communication channel that is experiencing multi-path distortion, the original transmitted signal can arrive at the receiver along with delayed versions of the transmitted signal. Typical delays between the first reception and the delayed receptions on a high frequency (HF) communication channel are 2–4 ms. FSK data modems combat the effects of this distortion by transmitting the sinusoids for symbol or baud durations, which are long in comparison to the delay distortion. For example: a 75 baud FSK modem will send one of two sinusoidal signals, depending on the data, for a duration of 13.3333 ms (milliseconds)( 1/75), typical HF delay distortion of 2–4 ms will only corrupt the first 15–30% of the received waveform.
Current receiver implementations assume a certain amount of multi-path distortion and disregard the portion of the received waveform, identified as the guard time, that is prone to this distortion. This would be, for the above example, the initial 2–4 ms of each transmitted symbol, which may be corrupted by a delayed version of the previous symbol. The disadvantage of this approach is that an implementation may assume too little or too much multi-path guard time. If too much multi-path guard time is assumed valuable signal energy is discounted from the receive process with a resulting higher bit error rate. If too little multi-path distortion protection is assumed, the distortion of previous symbols can corrupt the demodulation of the current symbol resulting in a higher bit error rate. Thus there is a need for a system to determine and apply an appropriate guard time for each received symbol at or near the signals symbol rate.
Therefore, it is an object of the present invention to obviate the deficiencies of the prior art and present a novel system and method for implementing an adaptive receiver that adjusts the amount of guard-time per symbol based on an estimate of the degree of multi-path distortion being experienced on the communications channel.
It is another of object of an embodiment of the present invention to present in a radio frequency receiver for receiving an M-ary symbol signal in a multi-path signal environment, a novel method of adaptively determining the guard time between successive received symbols. Embodiments of the method include predetermining N estimates of possible guard times Gn between 0 and T, receiving a transmitted symbol; and providing the received symbol to M×N parallel processing units Pmn. The method also includes multiplying the received waveform by a function Fm; in each of the plural parallel processing units Pmn and integrating the product over a lower limit of integration Gn and an upper limit of integration T in each of the plural parallel processing units Pmn to create a M×N outputs. An embodiment of the method also includes generating an estimate of multi-path distortion from the M×N outputs and selecting M parallel processing units Pmk based on the estimate of multi-path distortion, where k is a constant.
It is yet another object of an embodiment of the present invention to present in a radio frequency communication system a receiver for receiving an M-ary symbol signal in a multi-path distortion environment. Embodiments of the receiver including two or more sets of processing branches for processing a communication signal wherein each processing branch comprises a multiplier for multiplying the received signal by a function f and an integrator with a lower integration limit of g and an upper integration limit of T for integrating the product to produce an output. Wherein, for embodiments of the receiver, the function f is different for each processing branch within the respective set of processing branches and the lower integration limit g is different for each of the two or more sets of processing branches. Embodiments of the receiver additionally include an estimator connected to the output of each of the processing branches for creating an estimate of the multi-path distortion of the received signal and a selector; for providing the output of one of the sets of processing branches to a decision unit based on the estimate of the multi-path distortion. The decision units in embodiment of the receiver comprise logic circuits for determining the symbol transmitted.
It is still another object of an embodiment of the present invention to present in a radio frequency receiver a novel method of determining the multi-path distortion on a communication channel. An embodiment of the method includes determining the number of possible transmitted symbols and determining several hypothesized degrees of multi-path distortion where for each possible combination of possible transmitted symbols and hypothesized degree of multi-path distortion, the communication signal is integrated over a period defined by the respective hypothesized degree of multi-path distortion and symbol duration. Embodiments of the method include creating an estimate of the actual multi-path distortion from the results of integration and selecting the results of the integration determined by the hypothesized multi-path distortion most closely matching the estimate of multi-path distortion and determining the symbol from the selected integrations.
It is still yet another object of an embodiment of the present invention to present a novel improvement of a radio frequency receiver for receiving an M-ary signal over a multi-path communication channel including (a) a detector for each M possible signal symbols for processing the received signal as a function of each possible transmitted frequency, Fm and a guard time G, and (b) a decision processor for receiving the processed signals to make a symbol decision. The novel improvement includes N−1 additional detectors for each M possible signal symbols, wherein the N−1 additional detectors have different guard times Gn−1 different from each other and not equal to G. Embodiments of the improvement also include a multi-path estimator for providing an estimate of the multi-path distortion from the processed signals from the M detectors and M×(N−1) additional detectors; and, a metric selector for selecting M processed signals with a guard time g corresponding to the estimate of multi path distortion and providing the selected M processed signals to the decision processor.
These and many other objects and advantages of the present invention will be readily apparent to one skilled in the art to which the invention pertains from a perusal or the claims, the appended drawings, and the following detailed description of the preferred embodiments